a. Field of the Invention
This invention relates to a tree pruning machine and, more particularly, to the structure of a cutting unit for the tree pruning machine.
b. Description of the Prior Art
A tree pruning is necessary for the acceleration of the growth of standing trees, the cultivation of a high-grade timber without a knot, the prevention of a damage by harmful insects or by snow, the effective utilization of a forest or branch pruning. A tree pruning machine for cutting the branches of a standing tree while climbing the tree by its own force is known as a machine for automatically pruning the tree. The tree pruning machine of this type consists of a body having wheels for vertically climbing and descending the tree, and a cutting unit mounted on the body. An engine drives the wheels, and the cutting unit cuts the branches of the tree while the body climbs the tree. The cutting unit cuts the branches projecting from the standing tree from the roots of the branches substantially at the skin of the tree by a saw chain while turning around the tree, and necessitates the following requirements.
(i) The cutting unit does not unnecessarily deform at its frame due to its twisting even if the unit makes contact with a branch having large cutting resistance such as a thick branch or a hard branch.
(ii) Since the frame of the cutting unit slidably contacts the surface of the standing tree, the cutting unit may fail to cut the branch due to excessive friction or defects in the skin of the tree, the frame must slidably contact with the tree as little as possible.
(iii) Since a great load is produced when the frame of the cutting unit makes contact with the branch during cutting, the cutting unit has a structure for contacting with the branch during cutting as little as possible.
(iv) The cutting unit can cut the branch substantially at the surface of the standing tree so as not to allow the remaining branch to occur.
(v) The saw chain of the cutting unit can smoothly turn around the frame of the unit.
(vi) The saw chain of the cutting unit can move in the cutting direction without unreasonable force.
The abovementioned various requirements are all important for the cutting unit of the tree pruning machine, and the cutting unit which does not incorporate the above requirements cannot satisfactorily cut the branches of the tree. Furthermore, in order to prevent the top of the cutting unit from intruding into the branch, the cutting unit has at the top a branch sensing member for effectively interrupting the drive itself in the climbing direction when the sensing member senses the branch. In this case, the sensing member must reliably telescope as predetermined and must not disturb pruning due to the contact with the branch.